long term timers

Example 1:
Long-term Timers
The following program examples show two ways to create long-term timers
with standard TIM and CNT instructions.
Two TIM Instructions
In this example, two TIM instructions are combined to make a 30-minute
timer.


TIM and CNT Instructions

In this example, a TIM instruction and a CNT instruction are combined to make a 500-second timer.TIM 0001 generates a pulse every 5 s and CNT 0002 counts these pulses. The set value for this combination is the timer interval × counter SV. In this case, the timer SV would be 5 s x 100 = 500 s. With this combination, thelong-term timer’s PV is actually the PV of a counter, which is maintained through power interruptions.

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two stage counter

When an SV higher than 9999 is required, two counters can be combined as shown in the following example. In this case, two CNT instructions are combined to make a BCD counter with an SV of 20,000.

Planning your project

It is very important to plan your work in designing your PLC projects. identify your project requirements, inputs and outputs involve etc.

Today i will give you a free PLC programming on how to do a PLC controller for a water tank system. This is a typical exam question for PLC beginner.

The inputs involve are 1 start button addressed at 0.00 and 2 level switches addressed at 0.01 and 0.02 respectively. The one address at 0.01 will turn ON whenever the tank is empty thus it will switch ON the water valve (output) which is address at 10.00. The other level switch will turn ON whenever the tank is full and consequently will turn the water valve OFF. By doing this control, the water tank will not run empty. The water will fill into the tank whenever the empty switch is ON. Address 14.00 and 14.01 is just internal relay address to sequence the programming.

Click HERE for your ladder logic

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